1. Field of Invention
The invention relates to compact, low-cost, low power micro digitized pixel photodetector arrays with true photon counting capabilities. The proposed detector arrays, which are not commercially available, are based on low noise avalanche photodiode (APD) arrays with micro digitization, built-in optical pre-amplification and internal multiplication gain that enable photon counting in a semiconductor array.
2. Brief Description of Related Art
To date, photomultiplier tube (PMT) devices have been used for photon counting, however this is not a semiconductor approach that can be used for large format arrays. Moreover their reliability is poor, they are susceptible to magnetic fields and require high voltage power supplies. Currently, the semiconductor approach uses avalanche photodiode (APD) arrays, which have low gain with high excess noise and can only be operated for photon counting in the Geiger-mode.
For larger format arrays, which are necessary for enhanced spatial resolution, semiconductor approaches with their capability for monolithic integration and high volume, low cost production are necessary. However, the most common semiconductor approaches using positive-intrinsic-negative (PIN) diodes and avalanche photodiodes (APDs) are not suitable for photon counting applications. First, PIN diodes have either no optical or internal gain. Secondly, APDs have low internal gains (10-100) in the linear mode (i.e. below the breakdown voltage) and only marginally high (100-1000) in the non-linear mode (the so-called Geiger mode above the breakdown voltage). For true photon counting, the gain must be sufficiently high and constant so that the output signal produced by one photon can usually be distinguished from the dark noise. This means that the gain has to be greater than 10,000 in the steady state in order for the probability of detection to be greater than 70%.
In the Geiger-mode, APDs are often used as photon counting detectors by biasing them above the breakdown voltage where any incoming photon triggers an avalanche pulse whose pulse-width can be extremely long unless it is limited by external high resistance networks (passive quenching) or active quenching circuits. This need to quench Geiger-mode APDs limits the minimum pulse width to greater than 16 ns for passive quenching or to 8 ns for active quenching; a limitation which makes them unsuitable for the applications in which pulse width less than 1 ns is critical. In addition, because Geiger-mode APDs perform photon counting in a gated, nonlinear regime, they are susceptible to timing jitter and have limited dynamic range.
Implementation of this invention would result in a breakthrough in true photon counting semiconductor optical receivers for use in the next generation defense and commercial applications. They include: ladar, free space optical communications, position sensitive sensors, quantum encryption and computing, biomedical applications, metrology, bioluminescence and single molecule fluorescence microscopy, IR spectroscopy and photon correlation spectroscopy